The present invention relates to an electrophotographic copier, laser printer, facsimile apparatus or similar image forming apparatus and, more particularly, to an image forming apparatus of the type replenishing, when a toner end sensor indicates that the amount of toner remaining in a developing unit is less than a predetermined amount, fresh toner into the developing unit.
Conventional electrophotographic copiers, for example, include one which collects toner remaining in a photoconductive element after image transfer by a cleaning unit and constantly recirculates the collected toner to a developing unit for recycling it. In this type of copier, the developing unit is provided with a toner end sensor in the vicinity of a fresh toner inlet thereof. The toner end sensor is responsive to the amount of toner remaining in the developing unit. When the amount of toner around the fresh toner inlet decreases to below a predetermined amount, fresh toner is replenished into the developing unit in response to the resulting output of the sensor. At the same time, the collected toner is constantly recirculated into the developing unit via a used toner inlet. This, however, often brings about a problem that when the output of the sensor indicates a toner end condition, toner still exists in more than the predetermined amount around the used toner inlet. In such a condition, if fresh toner is replenished into the developing unit, the toner overflows the unit via the used toner inlet and thereby obstructs the recirculation of the used toner into the unit. This aggravates the aggregation of the toner which would lead to a granular defective image in the event of recycling.